Alonso lapped in one minute 16.973 seconds, with the Mediterranean principality bathed in bright sunshine as the track preparations for the season's glamour race roared into life.

British rookie Hamilton, leading Alonso by two points in the overall standings after four races, had been quickest early in the session.

However the 22-year-old had completed just 14 laps of the tight and twisty street circuit before he was sidelined in the pits by a broken starter motor.

The team said a piece of debris had been left in the gearbox, forcing mechanics to remove the floor of the car to fix the problem.

The Briton, who ended the session 0.628 seconds slower than Alonso, has been on the podium in the first four races of his Formula One career and has been tipped to become the first rookie to win in Monaco this weekend.

He is already the youngest driver to lead the world championship.

Hamilton has been a winner in Monaco in junior championships, triumphing last year in the GP2 support series while Alonso was dominant in the showcase race with Renault.

BMW Sauber's Nick Heidfeld was third fastest in practice, with Renault's Giancarlo Fisichella setting the fourth best time before crashing into the tyre wall at Ste Devote at the end of the pit straight.

Ferrari's Kimi Raikkonen, a winner with McLaren in 2005, was fifth fastest ahead of the Renault-powered Red Bull of Australian Mark Webber.

The session had earlier been halted for five minutes when Dutchman Christijan Albers's Spyker stopped near Tabac and had to be removed from the track.